Diffractive $\rho^{0}$ production at COMPASS experiment

The helicity structure of the diffractive leptoproduction of $\rho$ mesons, $\mu+N\rightarrow\mu+N+\rho$, is studied and preliminary results on several spin density matrix elements and $R=\sigma L/\sigma T$ for incoherent exclusive $\rho^{0}$ production from COMPASS 2002 data are presented. The elements $r^{04}_{00}$, $r^{04}_{1-1}$, $\Im mr^{3}_{1-1}$, $r^{1}_{1-1}$ and $R$ are measured at $\langle W\rangle =$ 10 GeV in a wide range of $Q^{2}$, 0.01 $< Q^{2} <$ 10 GeV$^{2}$. The data are consistent with a substantial increase of $R$ with $Q^{2}$ and a weak violation of SCHC, in agreement with other high energy experiments.